This invention relates to airsprings, particularly to an improved piston for use on rolling lobe airsprings utilizing a flexible member which forms a rolling meniscus or lobe on the exterior of the piston.
A wide range of specific load deflection requirements can be met by tailoring the piston contour. The tailored contour of the piston allows for a change in cross-sectional area over the axial run or stroke of the piston thus changing the effective area over which the air pressure acts. This changes the spring rate of the rolling lobe airspring over its working stroke range. The ability to contour the piston of a rolling lobe spring eliminates the need for external air reservoirs which are used with other types of airsprings to achieve low spring rates and natural frequencies.
Spring rate is directly proportional to the square of the effective area. Effective area is defined by the equation EQU A.sub.e =F.sub.s /P.sub.g
where
A.sub.e =effective area [m.sup.2 ]
F.sub.s =spring force in kN(10.sup.3 N)
P.sub.g =gauge pressure, in kPa(10.sup.3 N/m.sup.2).
Effective area can be approximated for a rolling lobe airspring by the equation: EQU A.sub.e =[.pi.(D+d).sup.2 (0.9)]/16
where
D=the flexible member working diameter and
d=Piston diameter.
A decrease in the effective area can be achieved by either decreasing the flexible member diameter or decreasing the piston diameter.
Contouring the piston of a rolling lobe spring is a means by which the designer finally shapes the total load/stroke curve. A positive taper hereinafter shall mean an increasing diameter toward the bottom of the piston. A positive taper produces a higher spring rate. A negative taper or back taper piston, where the effective area diminishes with stroke, produces the lowest spring rate.
There are practical limits to maximum and minimum piston diameters which relate to the flexible members diameter and its ability to form a meniscus as it rolls downward during the stroke of the piston. There are also limits to the angle of back-taper which involve the ability of a given flexible member to follow an extreme contour.
Most pistons are made by metal casting techniques or in particular applications by molding synthetic plastic resins to the desired shape. When extreme negative or positive tapers are employed in the piston design, the problem of manufacturing the piston in one piece becomes significant. When casting or injecting a piston, a split outer mold is typically used to form the outer periphery of the airspring piston and a core is positioned inside of the mold while the casting or injection is being accomplished for the purpose of controlling the wall thickness and interior dimensioning of the piston. After completion of the forming step, the core must be removed. The pistons with which this invention is concerned are hollow or cup-form pistons which have a larger diameter end and a smaller diameter end. The larger diameter end is always closed in this configuration. The smaller diameter end may be open or closed as desired.
Significant problems arise in the manufacturing of such piston configurations. The core may not be solid since after the casting or injection is completed the core cannot be removed by axially withdrawing it from the interior of the piston. This is due to the fact that the open end of the one piece piston design is smaller than the upper end, thus, a solid core cannot be axially removed without fracturing the wall.
Since the solid core is unusable in this kind of a piston several production approaches have been employed with resulting high cost and unsatisfactory rates of production of the piston. One method is to use a sand core which is preformed prior to the casting or molding operation and subsequent to the forming of the piston the sand core is destroyed and removed from the interior of the piston. This method is expensive and labor intensive since the disposable sand core is expensive and the removal of it takes labor time. A second approach is the use of a collapsible inner core which is formed in segments which slide or pantograph over each other to form a narrower core which may be withdrawn through the smaller diameter end of the piston subsequent to the forming operation. Both of these molding approaches have been found to be very unsatisfactory from a production efficiency and cost standpoint. The method of this invention is directed to the production of a two-piece piston to replace a single piece piston when the open end of the piston is of significantly smaller diameter than the other end.
An object of the invention is to provide a multiple piece piston which can be efficiently manufactured. It is a further object of the invention to provide a multiple piece piston which has significantly lighter weight than single piece piston of similar configuration. Yet another object of the invention is to provide a method of manufacturing an airspring piston which reduces the complexity and cost of the molds required for casting or injecting the piston body.